At present the light source used by liquid crystal display (LCD) of desktop computers and notebook computers, PDA, and Webpad generally is a cold cathode fluorescent lamp (CCFL) driven by a driving apparatus at a high voltage. Light is projected through a back light module to a back light plate to enable users to see the displaying picture.
The earlier back light module was widely used in commercial light boxes. These days with LCD industry flourishing, and high demand of luminous and full color requirements for the non-luminous LCD, the back light module is tightly coupled with the LCD and has become an indispensable item to provide even and high intensity lighting. It is a vital element in TFT LCD products. The back light module has gone through a lot of developments and progresses, such as from direct projection and side projection in the earlier time to using the light source of CCFL, LED and EL. The optical characteristics of the back light module also are reflected in the product features such as TFT LCD or STN LCD.
LCD TV or LCD display screen with a touch film requires a greater luminosity to compensate visual requirement. Refer to FIG. 1 for a conventional driving device for driving CCFL. It has a single pulse-width modulation (PWM) control unit 11 to receive voltage output from a power unit 10 and output a control signal to drive a driving unit 12 consisting of a P-MOSFET 120 and a N-MOSFET 121. The driving unit 12 then drives a piezoelectric unit 14 (such as ceramic piezoelectric transformer) and a load 15 (such as CCFL). The CCFL is ignited at a high voltage. When the current of the lamp increases, the luminosity also increases.
As previously discussed, when the back light luminosity and evenness of the back light module are not desirable, a common practice is to use multiple lamps. In such a condition, the number of PWM control unit 11, driving unit 12 and piezoelectric unit 14 required to ignite the lamps also increases. As a result, the size of the circuit board increases. Fabrication is more difficult and cost is higher.